1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device.
2. Description of the Related Art
There are known developing devices provided in image forming apparatuses such as laser printers or the like, which have a toner supplying roller (developing agent feed member) which supplies toner (developing agent) to a developing roller (developing agent bearing member), and scrapes off toner borne by the developing roller. The toner supplying roller is used primarily to prevent defective reproduction in solid images and ghosting. Defective reproduction in solid images is a phenomenon where density at the trailing edge of an image drops when the entire image is a 100% solid image. Ghosting is a phenomenon where, when a solid image with high density is formed and then a halftone image or solid white image is formed, traces of the solid image appear on the halftone image or solid white image.
There has been proposed in recent years a developing device from which the aforementioned toner supplying roller is omitted, to realize reduced size and costs of the developing device. Omitting the aforementioned toner supplying necessitates other measures to suppress defective reproduction in solid images and ghosting.
Japanese Patent Nos. 3272056 and 3162219 disclose a configuration of a developing device from which the toner supplying roller has been omitted, where dielectric portions and conductive portions coexist on the surface of the developing roller (developing agent bearing member), in regular or irregular distributions. In this configuration, a developing blade (regulating member) charges the dielectric portions by rubbing, either directly or with toner interposed therebetween, thus forming microfields at the adjoining portions of the dielectric portions and conductive portions. Toner is suctioned to the surface of the developing roller by the gradient force due to the microfields, and thus is borne thereby.
The developing device according to Japanese Patent No. 3272056 is configured such that
(−) toner<developing blade<dielectric portion (+) in triboelectric series, in a case where the charging polarity of the toner is negative polarity, for example. In such a configuration, the toner borne by the dielectric portion is powerfully electrostatically adhered to the dielectric portion, so regulation by the developing blade is difficult. Accordingly, the amount or toner coated on the developing roller when forming solid white images may be greater as compared to when forming solid images, and this difference in amount of toner coated may be manifested in the image as a ghost.
While the developing blade does serve to adjust the amount of toner coated, it does not serve to scrape off toner from the developing roller such as a toner supplying roller does. Accordingly, continuously outputting low-coverage images may result in melt-adhesion of toner to the developing roller, resulting in image defects. The lifetime of the developing device thus has to be set shorter, to avoid such image defects.